Relationship between thyroid function and seasonal reproductive activity in mares

The relationship between thyroid function and seasonal reproductive activity in mares was investigated by comparing plasma triiodothyronine and thyroxine concentrations in anoestrous mares and in mares with cyclic ovarian activity during the anovulatory season. In study 1, the diurnal changes in plasma triiodothyronine and thyroxine concentrations on a single day were compared in anoestrous and cyclic mares. The mean thyroxine concentrations were significantly higher (P < 0.001) in the cyclic mares than in the anoestrous mares, whereas mean triiodothyronine concentrations were similar in both groups. In study 2, plasma thyroid hormone concentrations were monitored in mares that foaled before the vernal equinox. Nine mares resumed cyclic ovarian activity shortly after parturition and ten mares remained in anoestrus. Mean plasma triiodothyronine and thyroxine concentrations were significantly lower in the first 60-90 days after foaling in the mares undergoing anoestrus than in the mares undergoing normal ovarian activity after parturition (P < 0.001). In these mares, the increases in triiodothyronine and thyroxine concentrations were associated with the first ovulation after parturition. In study 3, thyroid hormone concentrations were monitored in adult (n=14) and young (n=14) mares. Mean thyroxine concentrations were significantly lower in adult anoestrous mares during the anovulatory period than in cyclic mares and in anoestrous mares before and after the anovulatory period (P < 0.01). Mean plasma triiodothyronine concentrations were also significantly lower (P < 0.001) in anoestrous mares from December until March. There were no significant differences between the plasma triiodothyronine and thyroxine concentrations in young cyclic and anoestrous mares. This relationship between thyroid function and expression of seasonal reproductive activity does not indicate that the thyroid gland is involved in the control of seasonality in horses. The decreased triiodothyronine and thyroxine concentrations in anoestrous mares may be the result of a hypothalamic control similar to that described for seasonal reproductive activity.